Salmonella enterica (hereafter referred to simply as Salmonella) in production animals is predominantly a public health hazard. However, certain serovars can cause clinical disease in animals and mostly the young animals are at risk. The European Union (EU) is leading the global initiatives to reduce Salmonella in food animal production. Countries have a very diverse Salmonella situation, ranging from some countries negligible levels in the food animal production to other countries where Salmonella is highly prevalent. The dominant serovars of Salmonella vary between regions, countries and production species. Most serovars do not cause clinical disease in animals, but are more of public health concern.
Producers worldwide are now increasingly sharing the responsibility for the safety of food production. Egg and egg products are globally considered the major Salmonella risk food source for humans, and Salmonella control programmes in poultry have been shown successful and have resulted in less human salmonellosis. The methods of reduction of Salmonella will vary dependent on food animal production species, the agricultural infrastructure, prevalence in the animal population, resources and focus of the industry concerned.
There are no globally comparable estimates of the number of human cases of salmonellosis. Clinical cases of salmonellosis are not always diagnostically confirmed and reported, diagnostic procedures, methods and reporting vary between nations, making international comparisons very difficult, and the true number of Salmonella cases is likely very much higher in many countries than what is reported.
Control and surveillance programs in animals
Sampling programmes can strongly influence the estimated levels of contamination in herds and countries. It is important to assess the sampling programme and methodology used or to be used. Many programmes in poultry are based upon microbiological culture of environmental swabs. Several monitoring programmes in pig production are based upon a meat-juice ELISA test, where antibodies to Salmonella in the pigs are detected at slaughter. ELISA antibody detection is also used for Salmonella Dublin control in Denmark.
The challenges to reduce Salmonella in many production systems entail significant time and financial investments. Many of the existing animal husbandry systems, such as dairy and beer production, are not easy to sanitize, or prevent Salmonella from entering and spreading. Multiple intervention tools and management measures needs to be employed throughout the production pathway to lower the Salmonella risk to consumers. Biosecurity, animal purchase, animal flow, feed, management, nutrition, herd immunity are all very important factors and all influence levels of Salmonella in a herd.
Salmonella in Cattle
There are limited national surveillance programs for Salmonella in cattle compared to poultry or swine. Salmonella is very common on dairies world-wide (some exceptions such as the Nordic European countries).
Salmonella on dairies
There are several studies that give an indication of levels of Salmonella on dairies in USA. It has been estimated that from 27 to 31% of dairies across the United States are Salmonella infected. Bulk milk can often be contaminated by Salmonella in cow manure and environment on farm. Samples of bulk tank milk from 854 dairies in 21 states, taken as part of the National Animal Health Monitoring System Dairy 2002 survey, were analyzed for the presence of Salmonella and 12% of dairies were positive using rt-PCR techniques (screening for bacterial DNA) and 2.6% were positive with microbiological culture. In another study Salmonella was cultured in 6 % of bulk milk samples. In a study in California, where bulk milk was collected from 400 dairies at 8 different sampling occasions, Salmonella was found in the bulk milk between 10 to 20% of the sampling times. The most common serotypes were Montevideo, Typhimurium, Dublin and Give. One third of the Salmonella isolates recovered were multiple resistant to antibiotics. (Berge et al., 2007).
Studies have indicated that on dairies Salmonella shedding in cows can range from 0 to 99%, and 0 to 67% of pre-weaned calves and this can continue for several years. One study (Huston et al. 2002) found that Salmonella shedding in cows occurred in 31% of 105 Ohio dairies. The pre-weaned calf is exposed to Salmonella through calving, cow's milk and environment. A study of 33 dairy calf farms in California indicated that approx. 20% of day-old calves were shedding Salmonella in their faeces. The levels of Salmonella in the calves declined with age. It should be noted that this study the prevalence was based upon 0,1 gr faeces, and therefore the percentage of calves that are actually shedding Salmonella is most probably at least twice as high.
In Europe, a study of manure and slurry samples of 489 dairies in western France found Salmonella in 8 % of the herds and 2% of the dairies had multiple antibiotic resistant Salmonella (Lailler et al. 2005). Common serovars that have been reported in surveillance bovine meat samples in 2007 were Bredeney, Brandenburg, Derby, Dublin, Enteritidis, Hadar, Heidelberg, Indiana, Infantis, Kentucky, Livingstone, Panama, Rissen, Schwarzengrund, Typhimurium, and Virchow. (EFSA report)
Salmonella in dairy cattle
Salmonella infection can be present without clinical signs in the cows or calves. The majority of Salmonella serotypes cause no clinical signs in cows and calves and no in lactation. Animals that continually shed bacteria in their faeces (chronic shedders) are relatively common.
Some serovars of Salmonella cause serious disease in cattle, such as Typhimurium, Dublin and Newport. These three serovars have the ability to, not only cause diarrhoea, but also penetrate the intestines and go into the blood stream and cause disease in other organs of the body (systemic disease) such as lungs, brain, joints, liver and spleen. Furthermore they have become increasingly resistant to antibiotics, thereby making it more and more difficult to treat the disease resulting in increased deaths. Alternatives to antibiotics to treat Salmonella are continually and desperately sought.
Diagnosis in affected adults can be done with bacteriological culture of a faeces sample or a sample from an aborted calf. However, bacteriological culture of faeces may fail to detect bacteria present, so a negative sample does not necessarily mean that the animal does not have Salmonella. A proper post mortem examination is recommended. Finding Salmonella in otherwise healthy calves may not mean that the calf has disease, they may shed many serotypes of Salmonella without being sick. Culturing milk filters may indicate what serovars are present on the farm and may be present in the milk.
Calves are easily infected, especially in the early days of life when a protective gut flora has not been developed. Calves that have received insufficient colostrum, or not colostrum fed in the first few hours after calving are at increased risk of getting sick. Infected calves can shed the organism in faeces, urine, saliva, and nasal secretions, and the bacteria can easily spread in the calf environment and to calf workers and equipment. Salmonella can survive in the environment for months and moisture improves survival. Antimicrobial treatments may increase the risk of salmonellosis in calves, but are necessary if the calf gets a blood infection.
The best way to avoid salmonellosis in cattle is to prevent the bacteria from entering the herd and this can only be done with high levels of biosecurity. It was shown that a closed herd (not introducing cattle into the herd) was protective against Salmonella in the pre-weaned calves. The most important risk for introducing new strains of Salmonella on dairies was raising heifers on calf ranches with co-mingling of calves with other dairies.
Salmonella Typhimurium has spread all over the world and is usually resistant to multiple antimicrobials. A particular strain called Typhimurium DT104 rapidly spread across the globe and causes severe diarrhoea in cattle, swine, poultry, humans and pets. It has a characteristic resistance pattern to 5 antimicrobials referred to as 'ACSSuT', which stands for resistance to ampicillin, chloramphenicol, streptomycin, sulfonamides and tetracycline. These strains are also resistant to florfenicol (drug related to chloramphenicol). Some of the DT104 bacteria have also acquired resistance to cephalosporin and florquinolone antibiotics, and these are antibiotics that are important drugs to diarrhoea in humans. This has raised serious concern in the medical community, as we approaching a time where no antibiotics will be effective for treatment of human salmonellosis.
To avoid the spread of this bacteria the most important factor is to avoid purchasing cattle, avoid cattle markets, and other risks are direct contact with other farmers, sharing of equipment and contaminated slurry, cats and wild birds may transmit disease and contaminated areas.
In 2000 a new strain of Salmonella Newport rapidly spread across USA causing severe disease in cattle and humans. This Newport was shown to be resistant to at least 9 antibiotics. In many cases there were no antibiotics available to treat disease in cattle, and dairies suffered major losses, including adult cattle and calf diarrhoea and deaths.
This serovar is a cattle-adapted strain. S. Dublin causes a wide range of diseases in cattle, not just diarrhoea. It rarely causes disease in humans, but when it does the disease may be severe and may lead to death.
In adult cattle, acute and sub-acute forms of disease are seen. Acute disease may involve fever, dullness, decreased appetite and milk production, bloody diarrhoea and frequently death. The sub-acute form has milder symptoms and may lead to abortion of calves. Disease symptoms in calves are more variable and the disease signs usually start in calves 2-6 weeks of age, but may not be observed until later. The clinical signs are pasty diarrhoea that may become bloody, dehydration, septicaemia, pneumonia, joint disease, meningitis and death. Sometimes death may occur without diarrhoea.
Denmark is actively trying to eliminate S. Dublin from their bovine animals through a program where antibodies in the herd are detected through Elisa methods using bulk tank milk, individual milk samples or serum samples. Extensive internal and external biosecurity measures are implemented to reduce infection on farms and restrictions in trade are imposed in herds with high levels of infection.
Treatment and prevention of S. Dublin
Antibiotics and supportive treatment, particularly fluids either orally or in the vein, increase survival rates in calves and adults. S. Dublin bacteria are likely multiple antimicrobial resistant, therefore it is advisable to culture the bacteria and check for antibiotic sensitivity.
Adult animals, although they recover from the S. Dublin infection may become carriers of the bacteria and excrete it in faeces for long periods of time. These cows are likely to infect other animals and calves. Milk is a very good source of bacteria and disease is in calves fed infected raw milk. Isolation and separate handling of infected animals is recommended. Ensure that milk from infected cows is not fed to calves or that all milk fed to calves is pasteurized. Hygiene is very important, and special attention should be paid to calving areas.
Vaccines are available, however once S. Dublin has entered a herd, vaccination alone will not control the spread of infection. Good husbandry and hygiene is essential if control is to be achieved.
Public health on dairies
It is important to remember that Salmonella bacteria in bovine animals can cause disease in humans. Maintain a high level of personal protection and hygiene when working with cows and calves. Remember that visitors do not have the same immunity to the farm's bacteria as those regularly living and working on the farm. Do not drink raw milk unless regular tests for Salmonella are carried out and do not drink raw milk if there are any suspicions or confirmations of salmonellosis in animals on the farm.
Salmonella in beef cattle
Salmonella infections in feed lot cattle are most common in warm moist seasons. The disease is seen in calves with more severe diarrhoea and death loss than in typical scours outbreaks. Occasionally there are outbreaks of disease in older cattle. Preventing exposure to Salmonella is the key and biosecurity measures include preventing manure contamination of feed and water, being very careful with new animal introductions, being sure feed sources are clean and isolating cattle that have disease. Furthermore keeping cattle resistance and immunity high is equally important. Approaches include adequate nutritional management, special care at times of stress (shipping, weaning, calving, etc), and ensuring that newborn calves gets plenty of colostrum, improve gut health and prevent other diseases that can lower resistance.
Beef cattle contamination by Salmonella is a food safety risk. Movement of animals and environmentally-mediated transfer from animal-to-animal are effective ways to spread contamination between beef cattle. Salmonella can survive in pens and lairages for more than 24 hours, and thus the environment can be a reservoir of zoonotic agents. Samples from beef cattle from USA slaughter houses in 2007 indicated that the top three serovars were S. Montevideo, S. Anatum, and S. Newport. The hides of cattle may be contaminated by the faeces from other cattle during transport to the slaughter house. Even if only a few animals are shedding bacteria, they can contaminate many more during transport. The risk of transfer of Salmonella to a carcass from contaminated cattle hide is high. There is a risk that those bacteria multiply and grow in ground beef, and several foodborne disease outbreaks have been due to Salmonella in ground beef.
There are measures such as lowering cattle movement, keeping cattle dry and keeping animals in lairage for as short a time as possible which can reduce carcass contamination with Salmonella. In certain countries disinfecting carcass washes are utilized to reduce surface contamination.
Some points to consider in Salmonella control
- Ensure that all animals introduced into the herd are Salmonella free, quarantine new animals from herd.
- Avoid raising heifers off the dairy such as on a contract calf rearing facility. Ensure feed and feed stores are dry and protected from rodents and wild animals.
- Ensure that buildings, overalls, boots and farm equipment are mechanically cleaned and disinfected.
- Ensure that visitors have protective clean clothing and do not come directly from other farms.
- Segregate production categories.
- Mechanical cleaning and disinfection of calving area.
- Do not place sick animals (hospital pen) in proximity to calving area and fresh cows.
- Avoid flushing milk alleys and stalls with water to remove manure, use scraping device.
- Minimize stress, optimize environment and control viral diseases to optimize immunity.
- Ensure that every calf receives 4 liters of colostrum within 12 hours of birth.
- Feed calves fresh high quality feed, pasteurize fresh/waste milk.
- Avoid treating non-complicated calf diarrhoea with antibiotics.
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